The world is divided into things that look
designed (like birds and airliners) and things that don't (rocks and
mountains). Things that look designed are divided into those that really are
designed (submarines and tin openers) and those that aren't (sharks and
hedgehogs). The diagnostic of things that look (or are) designed is that their
parts are assembled in ways that are statistically improbable in a functional
direction. They do something well: for instance, fly.

Darwinian natural selection can produce an uncanny illusion of design. An
engineer would be hard put to decide whether a bird or a plane was the more
aerodynamically elegant.

So powerful is the illusion of design, it took
humanity until the mid-19th century to realize that it is an illusion. In
1859, Charles Darwin announced one of the greatest ideas ever to occur to a
human mind: cumulative evolution by natural selection. Living complexity is
indeed orders of magnitude too improbable to have come about by chance. But
only if we assume that all the luck has to come in one fell swoop. When
cascades of small chance steps accumulate, you can reach prodigious heights of
adaptive complexity. That cumulative build-up is evolution. Its guiding force
is natural selection.

Every living creature has ancestors, but only a fraction have descendants. All
inherit the genes of an unbroken sequence of successful ancestors, none of
whom died young and none of whom failed to reproduce. Genes that program
embryos to develop into adults who can successfully reproduce automatically
survive in the gene pool, at the expense of genes that fail. This is natural
selection at the gene level, and we notice its consequences at the organism
level. There has to be an ultimate source of new genetic variation, and it is
mutation. Copies of newly mutated genes are reshuffled through the gene pool
by sexual reproduction, and selection removes them from the pool in a way that
is non-random.

What makes for success in the business of life varies from species to species.
Some swim, some walk, some fly, some climb, some root themselves into the soil
and tilt green solar panels toward the sun. All this diversity stems from
successive branchings, starting from a single bacterium-like ancestor, which
lived between 3 and 4 billion years ago. Each branching event is called a
speciation: a breeding population splits into two, and they go their
separately evolving ways. Among sexually reproducing species, speciation is
said to have occurred when the two gene pools have separated so far that they
can no longer interbreed. Speciation begins by accident. When separation has
reached the stage where there is no interbreeding even without a geographical
barrier, we have the origin of a new species.

Natural selection is quintessentially non-random, yet it is lamentably often
miscalled random. This one mistake underlies much of the skeptical backlash
against evolution. Chance cannot explain life. Design is as bad an explanation
as chance because it raises bigger questions than it answers. Evolution by
natural selection is the only workable theory ever proposed that is capable of
explaining life, and it does so brilliantly.